A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements for transmitting an ultrasound beam and receiving a reflected beam from an object being studied. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Conventional ultrasound imaging systems may also use other focusing strategies. For example, the ultrasound imaging system may control the transducer elements to emit a plane wave. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay (or phase) of the applied pulses, the beam with its focal point can be moved to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound energy. The voltages produced at the receiving elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate delay and gain to the signal from each receiving element. For receive beam-forming, this is done in a dynamic manner in order to focus appropriately for the depth range in question.
Many conventional ultrasound imaging systems that are capable of acquiring 4D ultrasound data have included a two-dimensional transducer array (hereinafter a 2D transducer array). For purposes of this disclosure, a 2D transducer array is defined to include a transducer array where the center points of the transducer elements form a two-dimensional pattern. The two-dimensional pattern may follow a curved surface according to some embodiments. The transducer elements may be dimensionally generally similar in both length and width in a 2D transducer array, or have other aspect ratios. Additionally, a 2D transducer array has electronic focusing and steering. The 2D transducer array typically comprises a number of transducer elements arranged in a grid; the grid may have a square, rectangular, hexagonal, or other basis. By controlling the timing and amplitude of the elements in the 2D transducer array, it is possible to steer the transmitted ultrasound beam simultaneously in both an azimuth direction and in an elevation direction. The beam control can of course be derived in any chosen coordinate system. The use of a 2D transducer array allows the ultrasound transducer or probe to have greater flexibility and it enables greater accuracy in the acquisition of volumetric data.
During acquisition of volumetric ultrasound data it is often a challenge to get both sufficient frame rate and lateral resolution for the desired volume of interest. A common way to improve frame rate and/or lateral resolution is to acquire the entire volume as a set of sub-volumes. The acquisition of the sub-volumes may be gated to a physiological signal, such as an ECG signal. Acquiring gated volumetric ultrasound data of a patient's heart has two key challenges. The first challenge is that since it may take multiple heart cycles to acquire a complete volume of data, the display may show inconsistent data when the operator moves the probe in search of the correct view and good access. As the operator moves the probe, the image shown on the display may be generated from sub-volumes that were acquired at a previous probe position. In other words, the image on the display may not accurately reflect the real-time position of the probe. This may make it difficult for the operator to know if the new probe position is an improvement over one or more older probe positions. The second challenge is that if the probe and/or the heart move with respect to each other over several cardiac cycles, the ultrasound data may become prone to artifacts.
For these and other reasons an improved system and method of volumetric ultrasound imaging is desired.